Sheeting and chunking has been a problem in commercial, gas phase polyolefin production reactors for many years. The problem is characterized by the formation of solid masses of polymer on the walls of the reactor. These solid masses or polymer (the sheets) eventually become dislodged from the walls and fall into the reaction section, where they interfere with fluidization, block the product discharge port, and usually force a reactor shut-down for cleaning, any one of which can be termed a “discontinuity event”, which in general is a disruption in the continuous operation of a polymerization reactor. The terms “sheeting, chunking and/or fouling” while used synonymously herein, may describe different manifestations of similar problems, in each case they can lead to a reactor discontinuity event.
There are at least two distinct forms of sheeting that occur in gas phase reactors. The two forms (or types) of sheeting are described as wall sheets or dome sheets, depending on where they are formed in the reactor. Wall sheets are formed on the walls (generally vertical sections) of the reaction section. Dome sheets are formed much higher in the reactor, on the conical section of the dome, or on the hemi-spherical head on the top of the reactor (FIG. 1).
When sheeting occurs with Ziegler-Natta catalysts, it generally occurs in the lower section of the reactor and is referred to as wall sheeting. Ziegler-Natta catalysts are capable of forming dome sheets, but the occurrence is rare. But with metallocene catalysts, sheeting can occur in either location or both locations; that is, both wall sheeting and dome sheeting can occur.
Dome sheeting has been a particularly troublesome with metallocene catalyst systems. Typical metallocene compounds are generally described as containing one or more ligands capable of bonding to the transition metal atom, usually, cyclopentadienyl derived ligands or moieties, in combination with a transition metal selected from Group 4, 5 or 6 or from the lanthanide and actinide series of the Periodic Table of Elements.
One characteristic that makes it difficult to control sheeting with metallocene catalysts is their unpredictable static tendencies. For instance, EP 0 811 638 A2 describes metallocene catalysts as exhibiting sudden erratic static charge behavior that can appear after long periods of stable behavior.
As a result of the reactor discontinuity problems associated with using metallocene catalysts, various techniques have been developed that are said to result in improved operability. For example, various supporting procedures or methods for producing a metallocene catalyst system with reduced tendencies for fouling and better operability have been discussed in U.S. Pat. No. 5,283,218, which discloses the prepolymerization of a metallocene catalyst. U.S. Pat. Nos. 5,332,706 and 5,473,028 disclose a particular technique for forming a catalyst by “incipient impregnation.” U.S. Pat. Nos. 5,427,991 and 5,643,847 disclose the chemical bonding of non-coordinating anionic activators to supports. U.S. Pat. No. 5,492,975 discloses polymer bound metallocene catalyst systems. U.S. Pat. No. 5,661,095 discloses supporting a metallocene catalyst on a copolymer of an olefin and an unsaturated silane. PCT publication WO 97/06186 discloses removing inorganic and organic impurities after formation of the metallocene catalyst itself. WO 97/15602 discloses readily supportable metal complexes. WO 97/27224 discloses forming a supported transition metal compound in the presence of an unsaturated organic compound having at least one terminal double bond.
Others have discussed different process modifications for improving reactor continuity with metallocene catalysts and conventional Ziegler-Natta catalysts. For example, WO 97/14721 discloses the suppression of fines that can cause sheeting by adding an inert hydrocarbon to the reactor. U.S. Pat. No. 5,627,243 discloses a distributor plate for use in fluidized bed gas phase reactors. WO 96/08520 discloses avoiding the introduction of a scavenger into the reactor. U.S. Pat. No. 5,461,123, discloses using sound waves to reduce sheeting. U.S. Pat. No. 5,066,736, and EP-A1 0 549 252, disclose the introduction of an activity retarder to the reactor to reduce agglomerates. U.S. Pat. No. 5,610,244, discloses feeding make-up monomer directly into the reactor above the bed to avoid fouling and improve polymer quality. U.S. Pat. No. 5,126,414, discloses including an oligomer removal system for reducing distributor plate fouling and providing for polymers free of gels. There are various other known methods for improving operability including coating the polymerization equipment, controlling the polymerization rate, particularly on start-up, and reconfiguring the reactor design and injecting various agents into the reactor.
With respect to injecting various agents into the reactor, the use of antistatic agents as process “continuity additives” appear to hold promise and have been the subject of various publications. For example, EP 0 453 116 A1, discloses the introduction of antistatic agents to the reactor for reducing the amount of sheets and agglomerates. U.S. Pat. No. 4,012,574, discloses adding a surface-active compound having a perfluorocarbon group to the reactor to reduce fouling. WO 96/11961, discloses an antistatic agent for reducing fouling and sheeting in a gas, slurry or liquid pool polymerization process as a component of a supported catalyst system. U.S. Pat. Nos. 5,034,480 and 5,034,481, disclose a reaction product of a conventional Ziegler-Natta titanium catalyst with an antistatic agent to produce ultrahigh molecular weight ethylene polymers. For example, WO 97/46599, discloses the use of soluble metallocene catalysts in a gas phase process utilizing soluble metallocene catalysts that are fed into a lean zone in a polymerization reactor to produce stereoregular polymers. WO 97/46599 also discloses that the catalyst feedstream can contain antifoulants or antistatic agents such as ATMER® 163 (commercially available from ICI Specialty Chemicals, Baltimore, Md.).
U.S. Pat. No. 5,410,002, discloses using a conventional Ziegler-Natta titanium/magnesium supported catalyst system where a selection of antistatic agents are added directly to the reactor to reduce fouling. The amount of antistatic agent is described as depending on the granulometric distribution of the polymer or of the polymer being formed and one example of the antistatic agent is ATMER 163, but no method for dynamically adjusting or optimizing the amount of antistatic agent is disclosed.
U.S. Pat. No. 4,978,722, discloses a method for producing a propylene-alpha olefin block co-polymer in which one compound selected from the group consisting of an aromatic carboxylic acid ester, a phosphorous ester, an unsaturated dicarboxylic acid diester, a tertiary amine, and an amide are added to the gas phase of the polymerization reactor whereby the formation of low molecular weight polymer is suppressed and adhesion of polymer to the walls of the reactor is prevented. But there is no mention in U.S. Pat. No. 4,978,722 of measuring electrostatic activity nor is there any mention of a method to optimize the level of the compound that is added to prevent adhesion.
U.S. Pat. No. 5,026,795, discloses the addition of an antistatic agent with a liquid carrier to the polymerization zone in a gas phase polymerization reactor. Preferably, the antistatic agent is mixed with a diluent and introduced into the reactor by a carrier comprising the comonomer. The preferred antistatic agent disclosed is a mixture, which is marketed under the trademark STADIS® 450 by Octel Starreon and which contains a polysulfone, a polymeric polyamine, a sulfonic acid, and toluene. The amount of antistatic agent is disclosed to be very important. Specifically, there must be sufficient antistatic agent to avoid adhesion of the polymer to the reactor walls, but not so much that the catalyst is poisoned. U.S. Pat. No. 5,026,795 also discloses that the amount of the preferred antistatic agent is in the range of about 0.2 to 5 parts per million by weight (ppmw) of polymer produced; however, no method for optimizing the level of antistatic agent is disclosed based on measurable reactor conditions.
EP 0 811 638 A2, which is discussed above, discloses using a metallocene catalyst and an activating cocatalyst in a polymerization process in the presence of an antistatic agent, and also discloses the use of ATMER 163. EP 0 811 638 A2 also discloses various methods for introducing the antistatic agent, most preferably the antistatic agent is sprayed into the fluidized bed of the reactor. Another method generally disclosed is the addition of an antistatic agent with the supported or liquid catalyst stream so long as the catalysts are not severely affected or poisoned by the antistatic agent. EP 0 811 638 A2 includes examples in which the supported catalysts were slurried in mineral oil prior to being introduced to the reactor and the antistatic agent was introduced directly to the reactor when using the unsupported catalysts. Static was measured in the fluidized bed itself a few feet above the distributor plate. Preferably, the antistatic agent was added intermittently in response to a change such as a rising level of static electricity.
Although various methods have been developed to manage sheeting problems with metallocene catalysts and use of continuity additives has been investigated, the problem persists. One reason the problem persists is that the use of continuity additives can be accompanied by decreased catalyst efficiencies and productivities. Decreased catalyst efficiencies and catalyst productivities occur where additives injections are not matched precisely in regards to frequency and/or amount to arrest transient instances of reactor static, which can presage undesirable “reactor discontinuity events”.
Another reason sheeting problems with metallocene catalysts persist (and perhaps is the root-cause of the problem) is the lack of advanced warning of such events (Note: EP 0 811 638 A2). Most sheeting incidents with metallocene catalysis have occurred with little or no advanced indication by any of the previously known and/or used process instruments, including the conventional static probes used heretofore. (Conventional static probes are those probes that are located, as discussed herein, and as discussed in U.S. Pat. No. 4,855,370, ¼ to ¾ of a reactor diameter above the top of the distributor plate.) This lack of indication with conventional instruments by previously available measurable indicators has presented a significant challenge in efforts to troubleshoot and correct the sheeting problems (and the resultant reactor discontinuity) with metallocene catalyzed reactions.
One of the first descriptions of reactor sheeting was provided in U.S. Pat. No. 4,532,311. This patent was among the first to describe the important discovery that sheeting with Ziegler-Natta catalysts is the result of static electrification of the fluid bed. (not sure if it is a good idea to characterize the teachings unless it was explicit) A subsequent U.S. Pat. No. 4,855,370, combined the static probe of the '311 document with a means to control the level of static in the reactor. In the case of U.S. Pat. No. 4,855,370, the means to control static was water addition to the reactor (in the amount of 1 to 10 ppm of the ethylene feed). This process has proven effective for Ziegler-Natta catalysts, but has not been effective for metallocene catalyst reactions or reactors.
Understanding the causes of sheeting with metallocene catalysts has for many years been hampered by the lack of suitable instrumentation. In particular, the static probes (so called conventional static probes, located on the wall(s) of a reactor as noted above) used for Ziegler-Natta catalysts have not been effective for providing warning or notice of sheeting or chunking in metallocene catalyzed reactions and reactors utilizing such reactions. Wall and dome sheeting with metallocene catalysts usually occurs with no prior (or coincident) indication on the conventional reactor static probes. This can be seen in FIG. 7, which shows that there was virtually no response on the (conventional) reactor static probe(s) in a pilot plant prior to the wall sheeting incident with metallocene catalyst, compared to other static probe locations which did show a response (i.e. static above zero).
Thus, it would be advantageous to have a polymerization process utilizing metallocene catalysts, the process being capable of operating continuously with enhanced reactor operability (defined as the general absence of sheeting or chunks that might lead to reactor discontinuity events). It would also be highly advantageous to have a continuously operating polymerization process having more stable catalyst productivities and reduced fouling/sheeting tendencies based on readily measurable reactor conditions such as electrostatic activity at points in the reactor system, which need is answered by embodiments of the present invention.